1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device such as a thin film transistor.
2. Description of the Prior Art
A thin film transistor (hereinafter referred to as TFT) using amorphous, polycrystalline, microcrystalline semiconductors or a semiconductor obtained by mixing these semiconductors in place of a conventional single crystal semiconductor has been developed. The semiconductor materials listed as above are hereinafter collectively referred to as non-single crystal.
Of the non-single crystal semiconductors, an amorphous semiconductor material, particularly amorphous silicon (abbreviated as a-Si hereinafter) is frequently used when TFTs are formed in large quantities on a substrate having a large area from the advantages that its characteristics of semiconductor as a transistor are stable and a-Si having a large area can be formed.
In recent years, TFTs made of a-Si have been employed as transistors of an active matrix type liquid crystal display device.
Such a TFT also utilizes the advantages that a semiconductor film having a large area can be easily formed by the plasma CVD (chemical vapor deposition) method and at the same time, a silicon nitride (SiN.sub.x) film and an silicon oxide (SiO.sub.2) film to be a gate insulator film or a passivation film which constitutes the TFT can be continuously formed by only changing source gases in the same method.
A method of fabricating the above described TFT will be briefly described with reference to FIGS. 5A to 5H.
FIGS. 5A to 5H are cross sectional views showing the sequential steps of a method of fabricating an inverted staggered type TFT.
First, as shown in FIG. 5A, a thin metal film made of chromium (Cr), titanium (Ti) or the like is formed on an insulating transparent substrate 1 made of glass or the like, and this thin film is patterned using photoresist, to form a gate electrode 2.
Subsequently, as shown in FIG. 5B, a gate insulator film 3 made of SiO.sub.2, SiN.sub.x or the like, a semiconductor film 4 made of a-Si, and a passivation film 5 made of SiO.sub.2 or the like are sequentially deposited using the plasma CVD method or the like.
Then, as shown in FIG. 5C, the passivation film 5 is patterned by etching, leaving the above passivation film 5 only in a channel portion on the gate electrode 2.
Then, an n.sup.+ -type a-Si film 6 is deposited on the entire surface of the semiconductor film 4 including the passivation film 5 as an impurity doped semiconductor film having phosphorus (P) doped in large quantities into a-Si so as to make ohmic contact as shown in FIG. 5D and then, this deposited thin film is patterned, to form island regions 7 as shown in FIG. 5E.
Then, a metal film 8 made of aluminum (Al) or the like is formed on the n.sup.+ -type a-Si film 6 as shown in FIG. 5F, and then, the metal film 8 is etched, to form a source electrode 9 and a drain electrode 10 as shown in FIG. 5G.
Finally, as shown in FIG. 5H, the n.sup.+ -type a-Si film 6 on the passivation film 5 is etched away, thereby to form an inverted staggered type TFT.
In the above described fabricating method, the following four photoresist processes are required: the photoresist process for forming a gate electrode, the photoresist process for leaving a passivation film on a channel, the photoresist process for forming island regions and the photoresist process for forming source and drain electrodes. Accordingly, it is inevitable that the fabricating processes are complicated.
Furthermore, in the above described conventional fabricating method, the n.sup.+ -type a-Si film 6 is removed by plasma etching with the photoresist used in forming the source and drain electrodes 9 and 10 being left.
However, there are some problems. For example, the resist is cured by this plasma etching, to make it difficult to remove the resist from the surfaces of the electrodes 9 and 10, contributing to decreased yield.
Additionally, if the n.sup.+ -type a-Si film 6 is removed by plasma etching after removing the above described photoresist, there arise some difficulties this time. For example, a carbon polymer is produced on the surfaces of the electrodes 9 and 10.
Moreover, an article entitled "THE TRANSFORMATION OF a-Si: H INTO POLYCRYSTALLINE SILICON BY EXCIMER LASER IRRADIATION AND ITS APPLICATION TO TFTs", OPTOELECTRONICS Devices and Technologies, Vol. 1, No. 2, pp. 235 and 248, December, 1989 discloses the technique for turning a-Si into polycrystalline silicon (abbreviated as poly-Si hereinafter) by a laser beam and doping impurities into source and drain regions. This conventional technique, however, has the disadvantage of requiring the patterning process for leaving an impurity doped semiconductor layer having impurities previously doped in large quantities only in a portion where impurities are to be doped.